Significant skeletal muscle atrophy results in a loss of functional independence and quality of life, and interventions such as resistance exercise training are not fully effective in restoring muscle mass in elderly individuals. Age-related atrophy, as well as the diminished capacity for overload-induced hypertrophy, occur predominantly in fast-twitch fibers in aging skeletal muscle. Resting skeletal muscle protein synthesis rate and translational efficiency decline with age, and we have strong data showing that the phosphorylation of 5'-AMP- activated protein kinase (AMPK; which suppresses protein translation and synthesis) is upregulated with age in a fast-twitch-specific manner in resting and overloaded rat skeletal muscle. Moreover, AMPK phosphorylation was tightly and negatively correlated with the degree of overload-induced hypertrophy in fast-twitch muscles. The first aim of this investigation is to determine if elevated AMPK activity is responsible for the age-related decline in protein synthesis rate in resting fast-twitch skeletal muscles of old animals. Young adult (YA; 8 mo.), middle- aged (MA; 19 mo.), and old (O; 30 mo.) male Fisher 344 x Brown Norway rats will be given a single injection of saline or the AMPK activator AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside). In fast-twitch plantaris (PLT) muscles, AMPK phosphorylation/activity, rates of mixed and myofibrillar protein synthesis (via 3H- phenylalanine incorporation), AMPK-y subunits influencing AMPK activity, and potential signaling intermediates controlling protein translation downstream of AMPK will be measured. We hypothesize that AlCAR-stimulated AMPK-Q2 activity will suppress resting mixed and myofibrillar protein synthesis rates in fast-twitch plantaris muscles of young adult and middle-aged animals to the levels seen in old saline-treated animals. The second aim of this investigation is to determine if elevated AMPK activity is responsible for an age-related decline in protein synthesis rate and the age-related decline in hypertrophic capacity in overloaded fast-twitch skeletal muscles of old animals. YA, MA, and O male FBN rats will undergo unilateral surgical ablation of the gastrocnemius to chronically overload the fast-twitch PLT muscle for 7 days, during which AICAR or saline will be chronically administered. After 7 days, AMPK phosphorylation/activity, rates of mixed and myofibrillar protein synthesis, hypertrophy, AMPK-y subunits, and signaling intermediates will again be measured in the PLT. We hypothesize that AlCAR-stimulated AMPK-a2 activity will suppress resting mixed and myofibrillar protein synthesis rates and hypertrophy in overloaded fast-twitch PLT muscles of YA and MA animals to levels seen in O saline-treated animals. The long-term objectives of this investigation are to elucidate mechanisms underlying the atrophy and impaired hypertrophy of aging fast-twitch skeletal muscle, to provide research opportunities for undergraduate and graduate students, and to provide data on which to establish future R01 grant applications.